Prions are proteins that self-propagate an aggregated or amyloid-like structure that converts protein from its native state to this aggregated conformation. Proteins that contribute to protein misfolding disorders, like amyloid-2 in Alzheimer's disease, are now thought to misfold and aggregate in a prion-like fashion. When the mammalian prion protein, PrP, adopts its prion conformation (PrPSc), it forms an infectious amyloid that causes fatal neurodegenerative disorders called transmissible spongiform encephalopathies. Different conformations of PrPSc, referred to as prion strains, may be the underlying cause of both the species barrier as well as much of the variation in disease latency and pathology seen in prion diseases. Pathologic variation is also seen in other neurodegenerative disorders, suggesting distinct protein conformations may be involved in these diseases as well. Yet, it is still unclear how different self-propagating structures can originate from one polypeptide. Precisely how prion strains arise de novo and how a single prion protein can acquire and propagate different conformations is entirely unknown. The investigation of prions endogenous to the yeast Saccharomyces cerevisiae has provided much information about the structural differences possible among mammalian prion strains (called variants in yeast). The yeast translation termination factor Sup35p aggregates to form the [PSI+] prion. [PSI+] is an epigenetic factor that has many phenotypic consequences as it globally impacts translation and may be involved in regulating how a cell responds to fluctuating environments. Interestingly, the spontaneous conversion of [PSI+] requires the presence of another prion, [RNQ+], formed by Rnq1p. Both of these prions form different variants that can easily be monitored in the yeast cell. This allows us to examine what factors play a role in dictating how a protein can adopt distinct self-propagating conformations. The following specific aims will use an integrated molecular, genetic, and biochemical approach to investigate how different protein conformations could contribute to both the formation and phenotypic variation of age-related diseases: 1) Determine how [RNQ+] variants facilitate the formation of [PSI+]; 2) (A) Elucidate intragenic and extragenic mechanisms that regulate the formation and propagation of [RNQ+] variants; (B) Elucidate the key biochemical properties that define prion variants. Overall, this proposal will enhance our understanding of the physical and molecular basis of prion variants and provide insight into how alternative protein conformations can impact transmissibility between species and cause variation in disease progression of neurodegenerative disorders.